1. Field of the Invention
The present invention generally relates to devices for preventing electrostatic discharge, and more particularly to an adjustable, elastic static control wristband.
2. Description of the Prior Art
Electrostatic discharge, as well as the mere presence of a static electric field, can be extremely detrimental to sensitive electronic parts. This is particularly true of modern semiconductors and integrated circuits which may be degraded or destroyed by the buildup of static electricity in the workplace. Especially sensitive components can be severely affected by an electrical potential as small as 50 volts, yet the simple act of walking has been known to triboelectrically generate a potential of 30,000 volts or more.
The most common tool heretofore used in the battle against electrostatic discharge is a conductive tether which is designed to drain away excess electrostatic charge. One of the earliest grounding tethers is described in U.S. Pat. No. 3,015,754 issued to W. Legge, which illustrates a grounding strap for a human leg, with a tether connecting the strap to a conductive tread to be attached to the bottom of a shoe. Later devices incorporated a wristband, and there are several variations of the wristband and/or grounding tether.
The subject invention relates to an elastic static control wristband which has improved adjustability. Several patents disclose adjustment mechanisms in wrist or body straps. For example, the simplest adjustment device is a conventional buckle, such as that shown in U.S. Pat. No. 4,677,521. The primary disadvantage of such a device is that it must be manipulated each time the wristband is put on or taken off. This is also true for over-center snap type buckles such as that shown in French Patent 2,607,014 (see figure 3 of that patent). Moreover, the extent to which such buckles are adjustable is extremely limited. In other words, several different sizes must be provided for users with different sized wrists. It is also difficult to achieve a proper exact adjustment with these devices, since they provide only discrete adjustment settings. This is critical to the performance of the wristbands, since they must have good contact with the skin, and yet not be so tight to constrict circulation or otherwise be uncomfortable.
There are many alternatives to the standard buckle, such as the prior art clasp depicted in FIG. 1, which is also shown in U.S. Pat. No. 4,577,256 (see FIGS. 3 and 4 of that patent). This clasp utilizes a gate having a jam which firmly grasps the end of an elastic, conductive strap. The primary problem with this construction is that the loose (conductive) end of the strap which dangles from the clasp may come into contact with an electrical power source, presenting a hazard to the user. This construction is thus somewhat inconsistent with the statement in that patent (at column 4, lines 53-59) that the clasp body should be made of an antistatic material in order to avoid such an inadvertent contact with a power source. Of course, the free end of the strap may be cut off, but this creates the highly undesirable potential for unraveling of the material or release of small fibers into the work area which can damage the electronic components being handled. After cutting, it would also be impossible to readjust the wristband to fit a larger wrist. Similar clasp constructions are described in U.S. Pat. Nos. 4,639,825; 4,662,695; 4,680,668; 4,720,765; 4,755,144; and 4,782,425.
One variation on an adjustable wristband which overcomes several of the above-noted problems is shown in FIG. 2, this variation also being depicted in U.S. Pat. No. 4,577,256 (see FIG. 8 of that patent, and figures 10-11 which show a similar design). In this design, one end of the elastic, conductive strap passes through the clasp and is held loosely against the other portion of the strap by a guide. A gate/jam is still used to fix the effective length of the strap. One significant disadvantage in the design of FIG. 2 is that it is incompatible for use with dual-conductor type wristbands (such as that shown in FIG. 13 of U.S. Pat. No. 4,639,825). Furthermore, when this wristband is stretched and then relaxed over the wrist, the inner, overlapping portion may creep out from under the overlying strap, and may similarly create comfort problems when the overlapping portion of the strap curls onto itself.
Another prior art construction is shown in FIG. 3 which utilizes a strap material that has an insulative outer surface and a conductive inner surface. The strap material is folded back and held in place by a figure-8 ring. Such a wristband is produced by Light Year Industrial Co., and is similar to the construction shown in U.S. Pat. No. 4,816,964 (see FIG. 9 of that patent). A simpler version (with a plain conductive fabric strap) is also shown in U.S. Pat. No. 3,015,754. This construction is advantageous because it confines the free end of the strap within the loop formed by the rest of the strap. In other words, the free end is held between the strap and the skin of the user, and cannot inadvertently contact a power source. The problem with this construction, however, is that the loop back or folded portion of the strap reduces the effective length of the inner conductive surface. In other words, it is the outer insulative surface which contacts the skin along the folded area of the strap. On small wrists, this folded material can effectively cover all of the conductive inner surface rendering the fabric element of the wristband useless. Although this problem may be avoided by folding the strap outwardly in an opposite manner, this would then expose the inner conductive surface and re-create the aforementioned safety hazard. Another problem with this construction is that the folded portion will only stretch half as much as the unfolded portion for any given pulling force, meaning that the unfolded portion will undergo greater stresses in use and wear out sooner.
Another exemplary prior art adjustable wristband is shown in FIG. 4, which utilizes a D-ring to buckle two inelastic strap portions together. The straps are formed from a fabric which has a conductive portion along the center of the inner surface of the fabric. The free end is secured to the D-ring by a hook-and-loop fastener. Although this construction does not expose any conductive material, it suffers from the same drawbacks as the conventional buckle, i.e., it must be manipulated when put on and taken off, and the user may easily fit it loose for comfort, but resulting in poor skin contact.
Other variations of prior art adjustable wrist straps suffer one or more of the above disadvantages. For example, U.S. Pat. No. 4,847,729 discloses a SPEIDEL.TM.-type wristband (a chain of expandable metallic links) for use with static control. The only method of adjusting this kind of wristband, however, is to add a plurality of modified (inelastic) links, which is tedious, reduces the active expansion link percentage, and still only provides discrete expansion settings.
It would, therefore, be desirable and advantageous to design a static control wristband which allows the user to optimize the sizing of the wristband for a comfortable fit, without creating safety concerns or requiring the need of removal of excess conductive strap material. The design should also accommodate dual conductor wristband variations.